This invention is related to the development and use of environmentally friendly materials that can alleviate the growing environmental problem of excessive trash. This invention relates to the development of compositions that degrade rapidly by microorganisms. These compositions can be used in a variety of consumer disposable products. For example, the materials can be fabricated into films, fibers and molded parts for the manufacturing of disposable products such as diapers, sanitary napkins, adult incontinence products, and particularly tampon applicators.
Many natural polymers are known to degrade rapidly by microorganisms but most of them are not useful in fabricating articles because they lack the properties typical to plastic materials. In addition, they are not melt processable. One such material is natural starch. Being cheap and readily available, starch has become the leading component in many developments of biodegradable compositions. The challenge has always been to convert this heat sensitive powder into a melt processable composition that has good mechanical properties while maintaining its high rate of biodegradation.
Many natural and synthetic materials are also known to undergo a rapid digestion by microorganisms. Their chemical structures vary from hydrocarbons and carbohydrates to molecules with ester or amide linkages. Encyclopedia of Chemical Technology, Plastic, Environmentally Degradable, Supp. Vol., 626-68 (Kirk-Othmer, 3d ed.). Most of these materials are not useful in fabricating articles because they lack the mechanical properties typical to plastic materials, in addition to being difficult to process.
The use of starch as a low cost biodegradable filler in certain polymeric systems is known. A fairly high concentration of starch filler is required to achieve biodegradation. At the same time, the mechanical properties of the compositions deteriorate sharply with increasing starch content. The first breakthrough in making high starch content materials was the discovery that starch can be melt processed in the presence of water, heat, and pressure. See U.S. Pat. Nos. 4,133,784 and 4,337,181 ("the USDA patents").
The USDA patents describe the use of starch filled poly(ethylene-acrylic acid) (EAA) and polyethylene (PE) as a biodegradable film for packaging and agricultural mulch. These patents describe a process where the starch is first gelatinized in the presence of water at elevated temperatures. Under such conditions the highly hydrogen bonded natural structure is destroyed and melt flow is attained. The resulting mixture is then blended with EAA and PE. Large amounts of water are used (20%-50%) and the final starch content is as high as 60 wt. %, after the water is removed.
Tests have indicated that the biodegradation rate of the materials prepared according to these patents displayed unsatisfactory biodegradation properties. The injected molded parts containing PE/EAA with 40 wt. % starch did not degrade at all in the sewage system (0 to 5% weight loss) after 30 days. This time period was selected for testing because water treatment facilities typically had sewage in residence for about 30 days, but in modern facilities the in residence time is less than a week. However, sanitary protection devices may end up in septic tanks as well so to avoid accumulation and clogging in the plumbing it is reasonable to require a maximum 30 days for degradation.
A starch based blown film is also described in an article by Felix Otey. Ind. Eng. Chem. Res., 1987, 26, 1659-1663.
U.S. Pat. No. 3,949,145 describes a method to prepare polyvinyl alcohol films filled with starch for agricultural applications. This composition is highly water sensitive, making the composition highly unsuitable for disposable products such as diapers and sanitary napkins. Also, polyvinyl alcohol-containing samples degrade very slowly under simulated anaerobic sewage treatment conditions making compositions formulated according to this patent unsuitable for disposable products.
Injection molded starch capsules are described in U.S. Pat. No. 6,673,438. These capsules are made from pure starch in the presence of 5% to 30% water and heat. The structure of the strongly hydrogen bonded starch is believed to be destroyed under these conditions, creating a melt flow. The resulting material is stiff and extremely water sensitive, which make the compositions a poor candidate for biodegradable products such as disposable diapers and sanitary napkins.
Recent improvements in producing starch based biodegradable articles have been reported. See U.S. Pat. No. 4,673,438; U.K. Patent Number 2,190,093; and European Publications 282,451, 298,920, 304,401, 326,517, and 327,505.
U.S. Pat. No. 5,095,054 discloses polymer blends that can be transformed by heat and under pressure to form products having dimensional stability. These compositions comprise destructurized starch, and at least one substantially water insoluble polymer; the composition may optionally contain another water-insoluble polymer and a plasticizer. The starch used in these compositions has a relatively high water content that significantly modifies the starch's dimensional and mechanical properties; although the destructurized starch is thermoplastic, it is not water resistant. This starch composition is an unsuitable substrate for manufacturing disposable products such as tampon applicators, disposable diapers, and sanitary napkins.
U.S. Pat. No. 5,087,650 discloses biodegradable plastic compositions comprising unmodified starch and thermoplastic polymers. The starch too has a high water content that causes the starch to become destructurized when heated under pressure. This affects the starch's dimensional and mechanical properties and makes the compositions unsuitable for biodegradable thermoplastic compositions.
GB 2,190,093 describes starch compositions that consist of at least 72 wt. % starch and, optionally, a second hydrophilic polymer. The presence of at least 10 wt. % water is also essential in that process which makes the compositions unsuitable for consumer products.
PCT Application, International Publication Number WO91/02025, discloses a starch-polymer alloy comprising destructurized starch and an ethylene copolymer. Tests have indicated that compositions prepared according to this patent lacked the necessary biodegradation properties required to be suitable for consumer products.
The use of poly(3-hydroxybutyrate) (PHB) and poly (3-hydroxyvalerate) (PHV) copolymer blends are described in Dave. et al., Polym. Mater. Sci., 1990, 62, 231-35. Degradation studies of polycaprolactone/PHBV compositions yielded relatively low weight loss under lab enzymatic hydrolysis.
A molded tampon applicator from poly(3-hydroxybutyrate) and poly (3-hydroxyvalerate), a plasticizer, and a nucleating agent is described in U.S. Pat. No. 4,900,299, assigned to McNeil. Also disclosed is the optional use of a biodegradable filler as hydroxyalkyl cellulose or starch.
The use of starch filled polymers is the cheapest and most convenient route for producing functional biodegradable materials. But the starch content in such compositions must be very high (&gt;50 wt. %) to achieve a reasonable rate of biodegradation. Further, starch powder typically contains 10% to 40% by weight moisture. This starch is generally charged into an extruder where the combination of heat and pressure is sufficient to destructurize and melt the starch. Because starch becomes the major phase, the resulting materials are brittle and require further modifications.
Typically, a fairly high concentration of starch filler is required to achieve biodegradation. But conventional polymers such as polyethylene, polyurethane, etc. have a limited useful capacity for starch filler, that is, the mechanical properties of the compositions deteriorate sharply with increasing starch content. Further, microorganisms have been shown to selectively digest the starch without degrading the polymeric matrix.
The prior art discloses certain compositions that contain destructurized starch, which is a water soluble material and has a very limited use in consumer products. This starch is formed by exposing starch to heat and pressure in the presence of water, which breaks the hydrogen bonds between the starch molecules and renders the materials thermoplastic but soluble in cold water, and thus, unsuitable for consumer products. For this reason, other water insoluble polymers have been incorporated into such compositions, but this caused poor biodegradation.
Despite advances, there still remained a challenge to find a reasonable trade-off between water resistance and high rate of degradation. The present invention provides compositions that maintain a proper balance between mechanical properties, high biodegradation rate, processability, and low cost.